Stents are generally known. Indeed, the term “stent” has been used interchangeably with terms such as “intraluminal vascular graft” and “expandable prosthesis”. As used throughout this specification the term “stent” is intended to have a broad meaning and encompasses any expandable prosthetic device for implantation in a body passageway (e.g., a lumen or artery).
In the past ten years, the use of stents has attracted an increasing amount of attention due the potential of these devices to be used in certain cases, as an alternative to surgery. Generally, a stent is used to obtain and maintain the patency of the body passageway while maintaining the integrity of the passageway. As used in this specification, the term “body passageway” is intended to have a broad meaning and encompasses any duct (e.g., natural or iatrogenic) within the human body and can include a member selected from the group comprising: blood vessels, respiratory ducts, gastrointestinal ducts and the like.
Stent development has evolved to the point where the vast majority of currently available stents rely on controlled plastic deformation of the entire structure of the stent at the target body passageway so that only sufficient force to maintain the patency of the body passageway is applied during expansion of the stent.
Generally, in many of these systems, a stent, in association with a balloon, is delivered to the target area of the body passageway by a catheter system. Once the stent has been properly located (for example, for intravascular implantation the target area of the vessel can be filled with a contrast medium to facilitate visualization during fluoroscopy), the balloon is expanded thereby plastically deforming the entire stricture of the stent so that the latter is urged in place against the body passageway. As indicated above, the amount of force applied is at least that necessary to expand the stent (i.e. the applied the force exceeds the minimum force above which the stent material will undergo plastic deformation) while maintaining the patency of the body passageway. At this point, the balloon is deflated and withdrawn within the catheter, and is subsequently removed. Ideally, the stent will remain in place and maintain the target area of the body passageway substantially free of blockage (or narrowing).
See, for example, any of the following patents:    U.S. Pat. No. 4,733,665 (Palmaz),    U.S. Pat. No. 4,739,762 (Palmaz),    U.S. Pat. No. 4,800,882 (Gianturco),    U.S. Pat. No. 4,907,336 (Gianturco),    U.S. Pat. No. 5,035,706 (Gianturco et al.),    U.S. Pat. No. 5,037,392 (Hillstead),    U.S. Pat. No. 5,041,126 (Gianturco),    U.S. Pat. No. 5,102,417 (Palmaz),    U.S. Pat. No. 5,147,385 (Beck et al.),    U.S. Pat. No. 5,282,824 (Gianturco),    U.S. Pat. No. 5,316,023 (Palmaz et al.),    U.S. Pat. No. 5,755,771 (Penn et al.),    U.S. Pat. No. 5,906,640 (Penn et al.),    U.S. Pat. No. 6,217,608 (Penn et al.),    Canadian patent 1,239,755 (Wallsten), and    Canadian patent 1,245,527 (Gianturco et al.),for a discussion on previous stent designs and deployment systems.
To date, most stent development has focused on the so-called coronary stents. While a number of advances in art of coronary stent development have been made, there is room for improvement.
One area which has received little or no attention is the area of endovascular treatment of aortic disease. At this point it is useful to review diseases of the aorta.
Aortic diseases contribute to the high overall cardiovascular mortality. Relatively new imaging modalities (e.g., transesophageal echocardiography, magnetic resonance tomography, helical computed tomography, and electron beam computed tomography) have been introduced during the last decade. These new imaging techniques facilitate better and/or earlier diagnosis of aortic diseases, even in emergency situations. These new imaging techniques have had an effect on patient management during recent years allowing more rapid diagnosis and decision making.
Generally, aortic disease is caused by mechanisms which weaken the strength of the aortic wall, particularly, the aortic media. Such wall weakening leads to higher wall stress, which can induce aortic dilatation and aneurysm formation, eventually resulting in aortic dissection or rupture. The various categories of aortic disease are summarized in FIG. 1.
Diseases of the aorta are a significant problem in medicine. There are two general approaches: drug treatment and surgery. Drug treatment is used to lower blood pressure—this approach is disadvantageous since, at best, it modulates the effect of the disease while still leaving the patient at significant risk. Surgery is disadvantageous due to the high mortality and morbidity, even in centers of excellence. The increasing age of the population is resulting in an increased incidence of aortic disease as it is a degenerative disease. Further, aortic stiffness increases with age thereby reducing coronary and other artery perfusion.
There are three (3) indications of aortic disease which are regularly of clinical interest: (1) aortic dissection, (2) blunt chest trauma (with consequential trauma to the aorta), and (3) aortic sclerosis.
Aortic dissection is known to occur in approximately 15-20 cases/1 million inhabitants/year with a mortality of 50% in the first year and 5% per hour for the first 5 hours after the onset of symptoms. It results in a splitting of the aortic wall, a bleeding into the wall with formation of a true and false (new) lumen separated by a flap called “intima” with tear or “rupture point”. In patients with involvement of the ascending aorta, surgery is performed and drug treatment preferred in patients with involvement of the descending aorta. As stated above, despite surgeries mortality is still high. The main problem is the organ perfusion of the abdomen which results in shock and multiorgan failure. Relatively recent studies have demonstrated that intramural hemorrhage, intramural hematoma, and aortic ulcer may be signs of evolving dissections or dissection subtypes. Currently, the various forms of dissection may be classified as follows:    Class 1 (FIG. 2a): Classical aortic dissection with an intimal flap between true and false lumen;    Class 2 (FIG. 2b): Medial disruption with formation of intramural hematoma/hemorrhage;    Class 3 (FIG. 2c): Discrete/subtle dissection without hematoma, eccentric bulge at tear site;    Class 4 (FIG. 2d): Plaque rupture leading to aortic ulceration, penetrating aortic atherosclerotic ulcer with surrounding hematoma, usually subadventitial; and    Class 5 (FIG. 2e): Iatrogenic and traumatic dissection.
Each of these classes of dissection can be seen in their acute and chronic stages; chronic dissections are considered to be present if more than 14 days have elapsed since the acute event.
Classic Aortic Dissection (Class 1—FIG. 2a)
Acute aortic dissection is characterized by the rapid development of an intimal flap separating a true lumen and false lumen. Due to the pressure difference the true lumen is usually smaller than the false lumen. Intimal flap tears characterize communicating dissections. However, tears are not always found and non-communicating dissections are not uncommon. The dissection can spread from diseased segments of the aortic wall in an antegrate or retrograde fashion, involving side branches and causing other complications.
Intramural Hematoma/Hemorrhage (Class 2—FIG. 2b)
An intramural hematoma is believed to be the initial lesion in the majority of cases of cystic medial degeneration leading to aortic dissection in which the intimal tear seems to be secondary to preceding intramural dissection. Intramural hematoma may be the result of ruptured normal-appearing vasa vasorum which are not supported by the surrounding aortic media or the result of rupture of diseased vasa vasorum. As a dissecting hematoma extends along the aorta the weakened inner wall is subjected to the elongating force of the diastolic recoil. Differences in elasticity between the aortic fibrous adventitia and the inner more elastic media may play an additional role.
In autopsy studies, dissecting aneurysms without tears have been found in up to 12% of 311 autopsies. Others studies have reported an incidence of 4% in 505 cases. In a series of sudden deaths, 67% of patients with dissections did not have tears. The incidence of intramural hemorrhage and hematoma in patients with suspected aortic dissection, as observed by various new imaging techniques, seems to be in the range of 10-30%.
There are two distinct types of intramural hematoma and hemorrhage.
Type I intramural hematoma and hemorrhage shows a smooth inner aortic lumen, the diameter is usually less than 3.5 cm, and the wall thickness greater than 0.5 cm. Echo free spaces (seen echocardiographically) as a sign of intramural hematoma are found in only .quadrature. of the patients. The mean longitudinal extent of the hematoma is about 11 cm and the echo free spaces show minimal or no signs of flow.
Type 11 intramural hematoma and hemorrhage occurs in aortic arteriosclerosis. A rough inner aortic surface with severe aortic sclerosis is characteristic, the aorta is dilated to more than 3.5 cm, and calcium deposits are frequently found. Mean wall thickness is 1.3 cm with a range of from about 0.6 to about 4 cm, and echo free spaces are found in 70 degrees of the patients studied. The longitudinal extension has a similar range as in Type I hematoma, usually about 11 cm. Intramural hemorrhages are more often found in the descending than in the ascending aorta.
The fact that intramural hemorrhage and hematoma can lead to aortic dissection has only be demonstrated in follow-up studies. Acute aortic dissection as a consequence of intramural hemorrhage and hematoma develops in from about 28% to about 47% of the patients. It is associated with aortic rupture in from about 21% to about 47%; and regression is seen in about 10% of the patients.
Subtle-Discrete Aortic Dissection (Class 3—FIG. 2c)
The structural weakness can either lead to clinically undetected disease or minor forms of aortic dissection. Subtle dissection has been described as a partial stellate or linear tear of the vessel wall, covered by thrombus. After the partial tear forms a scar, this constellation is called abortive, discrete dissection. Partial ruptures of the inner layer of the aorta allow the blood to enter the already damaged media and thus cause dissection of the aortic wall, eventually leading to a second lumen within the wall, to a rupture or healing during follow-up.
Plague Rupture/Ulceration (Class 4—FIG. 2d)
Ulceration of atherosclerotic aortic plaques can lead to aortic dissection or aortic perforation. This was first observed by computed tomography. Innovations in imaging techniques (e.g., intravascular ultrasound, spiral computed tomography and magnetic resonance imaging) provide new insight. The ability to diagnose aortic ulceration has thereby been improved and further affects the descending thoracic aorta, as well as the abdominal aorta, and is usually not associated with an extensive longitudinal propagation or branch vessel compromise. Valvular, pericardial, or other vascular complications seem to be rare. The ulcer may penetrate beyond the internal border, often with an nipple-like projection with subjacent Type II intramural hematoma formation. The continuous erosion of the atherosclerotic plaque may eventually violate the internal elastic membrane. False aneurysms, aortic rupture or dissections may occur.
Aortic sclerosis is normally divided into four grades from thickening of the intima (Grade I) up to the development of free floating thrombi (Grade IV) with the danger of embolism. In elderly patients, the incidence of the Grade IV aortic sclerosis is increasing. This has lead to a significant occurrence of stroke in patients. Thus, if a treatment of aortic sclerosis Grade IV with thrombi free floating in the aortic lumen could be developed, this would likely obviate or mitigate the consequential occurrence of stroke.
Currently, there is no reliable treatment approach for aortic sclerosis particularly the Grade IV type. Anticoagulation is a known approach, however this treatment must be accepted with the danger of hemorrhagic strokes, particularly in the older patients Further, the therapy is very difficult to monitor. Surgery is very complicated and has a high mortality and morbidity. Currently, surgery is not seen as a desirable alternative to anticoagulation therapy.
Traumatic/Iatrogenic Aortic Dissection (Class 5—FIG. 2e)
Blunt chest trauma usually causes dissection of the ascending aorta and/or the region of the ligamentum Botalli at the aortic isthmus. Iatrogenic dissection of the aorta may rarely occur during heart catheterization. It is regularly seen following angioplasty of an aortic coarctation, but can also be observed after cross clamping of the aorta and after the use of intraaortic balloon pumping. Most catheter-induced dissections are retrograde dissections. They will usually decrease in size as the false lumen thromboses. Proximal progression of the coronary dissection into the aortic root may be observed. In blunt chest trauma, the large acceleration of the aorta is leading to an intimal, medial or transsection of the aorta particularly at the adjunction at the aortic arch and the descending aorta (15-20% of blunt chest trauma cases are related to aortic injury). As a consequence of this blunt chest trauma, mediastinal hematoma can occur with abrupt death of the patient. The blunt chest trauma is known to occur in accidents involving heavy motorcycles and cars, as well as in other chest traumas. The diagnosis is verge difficult but has been improved by transesophageal echocardiography. Typically, the damage to the aorta is limited to a small portion comprising 3 cm-5 cm of the aorta. Conventionally, surgery was the only treatment to stabilize these patients. A mortality rate of 90% has been seen if surgery was not timely preformed. Even if surgery was timely performed, there is a significant mortality rate.
Most prior art attempts to improve surgical techniques to treat aortic dissection have not be particularly successful.
It is also worth pointing out that the so-call “stent grafts” are not well suited for treating diseases of the aorta. Specifically, a conventional stent graft is generally of a fixed longitudinal length. Since the anatomy of each patient is different and the overall longitudinal length of the aortic or other endoluminal disease condition is variable, the stent graft should be of a specific or customized longitudinal length so as to minimize the occurrence side branch blockage. This is inconvenient and requires inventory stocking of a number of stent grafts having a variety of different longitudinal lengths to have devices on hand for use in most situations.
Thus, despite the advances made in the art, there is still a need for an endovascular prosthesis capable obviates or mitigates at least one of the above-mentioned disadvantages of the prior art. Specifically it would be desirable to have an endovascular prosthesis whose longitudinal length could be adjusted in vivo by the physician during implanted of the prosthesis.